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LM1279 データシート(PDF) 8 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM1279
部品情報  110 MHz RGB Video Amplifier System with OSD
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM1279 データシート(HTML) 8 Page - National Semiconductor (TI)

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Functional Description (Continued)
channel has its own independent control pin with the 0V to
4V control range. An input of 4V give no attenuation, and an
input of 0V gives the full 12 dB attenuation.
The output of the drive attenuator stage goes to the inverting
input of A2. Since this is the second inversion stage, the out-
put of A2 will be the non-inverted video signal. Note that the
output of gm1 goes to the non-inverting input of A2. Also note
that the output of A2 goes to the inverting input of gm1. This
is the feedback for the clamp circuitry. The output stage of A2
is an exact duplicate of the video output through A3. If a
390
Ω load impedance is used at the video output, then the
black level at the output stage will accurately track the output
of A2. The other input to gm1 is the desired black level output
of the LM1279. Since the LM1279 has a fixed black level out-
put, the non-inverting inputs to gm1 in all three channels go
to a fixed 1.35V internal reference. This sets the black level
output to a nominal 1.35V. gm1 acts like a sample and hold
amplifier. Once the sandcastle sync exceeds 3.6V gm1 is ac-
tivated, driving the input of A2 to a level where the video out-
put will be 1.35V. For proper DC restoration it is important
that gm1 be activated only during the horizontal flyback time
when the video is at the black level. gm1 also charges the
clamp cap to the correct voltage to maintain a 1.35V black
level at the video output. When gm1 is turned off the voltage
stored on the clamp cap will maintain the correct black level
during the active video, thus restoring the DC level for a
1.35V black level.
The input of A3 receives the output from A2. The video chan-
nel of A3 is a duplication of the output stage to A2. As men-
tioned in the previous paragraph this is done so that the DC
restoration can be done at the A2 stage. A3 also receives the
OSD input and a sandcastle input for blanking. By doing DC
restoration at the A2 stage, OSD or blanking can be acti-
vated at the output stage during the time DC restoration is
being done at A2. There is an interface circuit between the
sandcastle input and the A3 output stages. This interface cir-
cuit will activate the blanking if the sandcastle sync input is
between 1.7V and 6.0V. The blanking mode will force the
output down to a level of about 0.1V. This is a blacker-than-
black level and can be used for blanking at the cathodes of
the CRT.
Once the sandcastle exceeds 6.5V, then the output will no
longer be in the blanked mode, but DC restoration is still be-
ing done on the video signal.
The OSD signal goes into a special interface circuit. The out-
put of this circuit will drive the output of A3 to either an OSD
black level or to about 2.4V above the video black level
(OSD white level). The OSD black level is about 300 mV be-
low the video black level. This guarantees that if the OSD
signal is not activated for a particular channel, then its output
will be slightly below the cutoff level. If an OSD input is re-
ceived in a particular channel, then the video output will be at
the OSD white level. The OSD mode is automatically acti-
vated if there is only one OSD signal to any of the video
channels. This OSD control circuit will allow any color, ex-
cept black, during the OSD mode. This also saves the need
for a special signal to switch into the OSD mode. Remember
that at least one OSD input must be high to enable the OSD
mode, therefore black can’t be used in the OSD window.
Sandcastle Sync
This special sync signal is used to allow for a 20-pin OSD
video pre-amp with all the desired controls. By using a sand-
castle sync, both clamping and blanking can be activated
from the same pin. Figure 4 shows the sandcastle sync sig-
nal. There are four possible modes of operation with the
Sandcastle pulse. These modes are:
1.
Inactive Region
2.
Blanking and no Clamping
3.
Blanking and Clamping
4.
Clamping and no Blanking
Figure 4 also shows the voltage levels where the LM1279
switches from one mode to the other mode. As an example
the LM1279 will switch from the inactive mode to the blank-
ing and no clamping mode between 1.2V and 1.7V. For
proper operation the inactive input must be safely below the
1.2V level. The blanking with no clamping pulse must be
safely above 1.7V and below 2.8V. Blanking and clamping
must be between 3.6V and 5.8V. Clamping and no blanking
must be above 6.5V with the maximum voltage being limited
by V
CC.
If the monitor designer desires to blank at the cathode, then
he would go into the blanking and no clamping mode for
most of the flyback period. During this period it is also neces-
sary to do DC restoration. During this time the LM1279
should be operated in the blanking and clamping mode. In
this mode DC restoration is done without interfering with
blanking.
In some designs the horizontal phase shift capability of the
monitor is very large. In these designs the video can be
moved so the flyback period can be displayed during the ac-
tive trace period for the video. Now the clamping could be
done during the normal video sweep time. During this period
clamping with blanking will give a black bar on the CRT
screen. This is not a normal operating mode of the monitor,
but the monitor designer still may prefer not to display this
black bar. Under this condition the clamp pulse must be
above 6.5V.
A simple two transistor sandcastle generator is covered in
separate application note. This circuit will generate all four
states for the sandcastle sync, including the clamp with no
blanking when the clamping function occurs during the pe-
riod for active video. The switching time between the inactive
region and the clamp only region must be less than 30 ns if
complete elimination of any blanking pulse is required in an
application.
Applications of the LM1279
Two demonstration boards are available to evaluate the
LM1279. One board is the pre-amp demonstration board.
This board was used for testing and characterizing the
LM1279. The schematic for this board is shown in
Figure 5
and the printed circuit layout for this board is shown in
Figure
7. The other board is a complete video channel neck board
that can be directly plugged into the CRT socket. The sche-
matic for this board is shown in
Figure 10 and the printed cir-
cuit layout is shown in
Figure 11. The CRT driver used on
this board is the LM2407. Any of National’s monolithic CRT
drivers can be used in this board, but the LM2407 is consid-
ered the best match to the LM1279 based on cost and per-
formance.
Some important notes on
Figure 5. All three video inputs
have a 75
Ω terminating resistor for a 75Ω video system. This
is the normal video impedance of the video from a computer
system. It is possible to also have a 50
Ω system, then R1,
R3, and R5 would be changed to 50
Ω. R2, R4, and R6 are in
series with the video inputs of the LM1279. These three 30
resistors are required to protect the IC from any sudden volt-
www.national.com
8


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